Bi-directional reflectance of a varnished painting: Part 2: Comparison between the effects of the refractive indices, of the surface states and of the absorption of the varnish – experiments and simulations

This study, based on diffuse reflectance spectrometry compared with simulations, shows that three parameters influence the visual aspect of a painting when a varnish is applied on it. The difference between the refractive indices of air, varnish and painting induces a negligible effect. The levelling of the painted surface by a varnish is more consequent. It induces a vertical downwards translation of the reflectance spectrum of the painting. Absorption by the varnish occurs for small wavelengths, under 450 nm. It induces a second decrease of the spectrum in this range. The refractive indices, the surface state modification due to levelling and the absorption law as a function of the wavelength have been evaluated. These results lead to a successful agreement between simulated and measured spectra. Different paintings and varnishes have been investigated and an example of application is given for an old painting.